Start/enable OVS on neutron ovs agent nodes

Start and enable the openvswitch service during host prep for hosts
running the neutron ovs agent.

Note: this is pre-stein only as stein uses systemd dependencies to make
sure openvswitch is running when needed.

Change-Id: I5cea8cf283df45c965017e444a8363278666896c
Closes-Bug: #1804264
(cherry picked from commit a483d0b53c)

docker/services/neutron-ovs-agent.yaml

@@ -179,6 +179,11 @@ outputs:
         list_concat:
           - {get_attr: [NeutronLogging, host_prep_tasks]}
           -
+            - name: ensure openvswitch service is enabled
+              service:
+                name: openvswitch
+                state: started
+                enabled: yes
             - block:
               - name: Copy in cleanup script
                 copy: